Aerogels have various applications due to their high surface area and low densities. However, creating various material aerogels has remained a challenge. For example, to date there are only limited types of materials that can be made into “aerogel” structures. These include metal oxide aerogels (e.g., SiO2, Al2O3), carbon material aerogels (such as carbon, carbon nanotubes (CNTs), graphene), and more recently semiconducting chalcogenide aerogels (e.g., CdS, CdSe, PbTe).
One reason that the type of usable materials to form aerogels is limited is the challenge of forming the starting “gel.” Most aerogels are obtained through a sol-gel process with a suitable gelling agent precursor. For example, in the case of SiO2 aerogel, a liquid alcohol (e.g., ethanol) is mixed with a silicon alkoxide precursor, (e.g. tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS)). A hydrolysis reaction forms particles of silicon dioxide, which may form a sol solution. The oxide suspension then undergoes condensation reactions, which result in the creation of metal oxide bridges (M-O-M bridges or M-OH-M bridges) linking the dispersed colloidal particles. When this interlinking has stopped the flow of liquid within the material, a gel is made. Carbon aerogels are made by subjecting gel precursor to supercritical drying and subsequent pyrolysis of an RF aerogel at high temperature. Because this cross-linking reaction is specific only to a selected group of materials, the number of materials that may be used to form aerogels is limited.